June 2008

Rev 2

1/22

AN863

APPLICATION NOTE

Improved sensorless control with the ST62 MCU

for universal motor

The universal motor is today the most widely used motor in home appliances (vacuum

cleaner, washer, hand tool, food processor...). Three modes of operation exist:

– in many cases, it is directly connected to the mains, without any speed adjustment.

More and more, however, the decreasing cost of electronics allows to include an adjustable

speed drive in the appliance. The control can be open loop or closed loop:

– in open loop mode, the speed is adjusted by the user but not regulated: it decreases when

the load increases. This is the case in many vacuum cleaners for example.

– in closed loop mode, the speed is regulated. This mode is used when the speed must be

accurately kept at a given value, in washers for example. This mode requires to add a speed

sensor on the motor shaft. Such a sensor is usually a tacho generator or magnet sensor.

The drawbacks of the speed sensor are many, but all boil down to higher cost and lower re-

liability, not to mention the extra bulk needed to accommodate the sensor inside the plastic

housing of a drill for example.

This document describes a speed regulator without sensor: the speed sensing is performed

indirectly by the ST6220, low-cost 8-bit microcontroller, measuring the motor current. Perform-

ance results are given, which are in line with the need of many home appliances.

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